This application is based on Japanese Patent Application No. 11-287933 filed October 8, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a rear driving apparatus for driving an automotive vehicle in the reverse direction, and more particularly to technologies associated with a synthesizing/distributing device of gear type used in the rear driving apparatus.
2. Discussion of Related Art
There is known a hybrid automotive vehicle equipped with a drive system including (a) an engine operated by combustion of a fuel to produce a drive force, (b) an electric motor, (c) an output member operatively connected to drive wheels, (d) a planetary gear device associated with the engine, electric motor and output member, for synthesizing and distributing a drive force or forces, and (e) a plurality of frictional coupling devices such as clutches and brakes for selectively connecting and disconnecting rotary elements of the planetary gear device and the output member to and from each other or a stationary member. An example of such a hybrid vehicle drive system is disclosed in JP-A-9-37411, wherein various drive modes are selectively established for driving the vehicle, by controlling the operating states of the clutches and brakes. For instance, the drive modes include a motor-drive mode in which the vehicle is driven by the electric motor as a drive power source, and an engine-drive mode in which the vehicle is driven by the engine as the vehicle drive source.
In a hybrid vehicle as described above, only the electric motor is generally used as the drive power source to drive the vehicle in the reverse direction. However, the drive force produced by the electric motor may be insufficient when the vehicle is driven in the reverse direction on an uphill. In particular, the drive torque produced by the electric motor is limited when the amount of electric energy stored in an electric energy storing device is reduced below a certain lower limit.
The assignee of the present application disclosed in JP-A-2000-92612 (published on Mar. 31, 2000 after the present invention was made) a hybrid vehicle equipped with a drive system including (a) an engine, (b) an electric motor, (c) an output member operatively connected to drive wheels, (d) a double-pinion type planetary gear device having a sun gear connected to the engine, a carrier connected to the electric motor, and a ring gear, (e) a brake for fixing the ring gear to a stationary housing, (f) a first clutch for connecting the carrier to the output member, (g) a second clutch for connecting the ring gear to the output member, and (h) a control device adapted to engage the first clutch and operate the electric motor in the reverse direction, for rotating the drive wheel through the output member for driving the vehicle in the reverse direction, the control device being further adapted to engage the brake and operate the engine as needed, so as to provide an assisting drive torque for driving the vehicle in the reverse direction. The drive system of this hybrid vehicle is capable of providing a sufficiently large drive force for driving the vehicle in the reverse direction. However, the drive system suffers from a relatively large variation in the rear drive force upon an engaging action of the brake, resulting in a risk of a stall of the engine when the running speed of the vehicle is relatively low. Thus, the drive system disclosed in JP-A-2000-92612 is not practically available.
It is therefore an object of the present invention to provide a rear driving apparatus for driving an automotive vehicle in the reverse direction, which apparatus permits the engine to be operated to provide an assisting rear drive torque through a synthesizing/distributing device of a gear type even at a relatively low running speed of the vehicle, while minimizing a variable in the rear drive force.
The above object may be achieved according to a first aspect of the present invention, which provides a vehicle rear driving apparatus for driving an automotive vehicle in a reverse direction, the automotive vehicle having a drive power source, an output member operatively connected to a drive wheel of the vehicle for driving the vehicle, a synthesizing/distributing device of gear type including a housing, a first rotary element, a second rotary element and a third rotary element, the first and second rotary elements being rotated in respective opposite directions when the third rotary element is stationary, and connected to the drive power source and the output member, respectively, and a brake for fixing the third rotary element to the housing, the vehicle rear driving apparatus being characterized in that the brake is a frictional coupling device capable of effecting a slipping engagement, and that reverse friction-drive control means is provided, the reverse friction-drive control means being operable when the first rotary element is rotated in a forward direction with an operation of the drive power source, while the third rotary element is rotated in the forward direction with a rotary motion of the first rotary element, the reverse friction-drive control means controlling the brake to effect the slipping engagement so as to restrict a rotary motion of the third rotary element, for thereby causing a drive torque to act on the second rotary element in a reverse direction to drive the vehicle in the reverse direction.
In the vehicle rear driving apparatus constructed according to a first aspect of this invention, the reverse friction-drive control means is operated when the first rotary element is rotated in the forward direction with an operation of the drive power source while the third rotary element is rotated in the forward direction with a rotary motion of the first rotary element. The reverse friction-drive control means is adapted to effect the slipping engagement so as to restrict a rotary motion of the third rotary element, for thereby causing a drive torque to act on the second rotary element in a reverse direction to drive the vehicle in the reverse direction. Thus, the vehicle can be driven with a sufficiently large drive force by utilizing an engine, for example. Further, the slipping engagement of the brake makes it possible to reduce a variation in the vehicle drive force, and permits the vehicle to be driven in the reverse direction even at a relatively low speed while maintaining a relatively high operating speed of the drive power source such as an engine.
The synthesizing/distributing device of gear type may be a planetary gear device of single- or double-pinion type, or differential gear device of bevel gear type. The frictionally coupling brake capable of effecting a slipping engagement is preferably a hydraulically operated friction brake of multiple-disc type. However, the brake may be an electromagnetically operated brake. A clutch or clutches, or other brake or brakes may be disposed between the second rotary element and the output member.
The drive power source preferably includes an engine which is operated by combustion of a fuel, and may include any other type of power source such as an electric motor. The vehicle rear driving apparatus according to the first aspect of the invention need not comprise reverse-motor-drive control means for driving the vehicle in the reverse direction with only an electric motor while the brake is held in the released state, as described below with respect to a second aspect of the invention.
The vehicle rear driving apparatus according to the first aspect of the invention described above is advantageously applicable to a hybrid vehicle wherein the drive power source includes an engine operable by combustion of a fuel, and an electric motor, and the synthesizing/distributing device comprises a planetary gear device including a sun gear as the first rotary element, a carrier as the second rotary element, and a ring gear as the third rotary element, the engine being connected to the sun gear while the electric motor being connected to the carrier. In this form of the invention, the brake is operable to fix the ring gear to the housing, and the apparatus includes a first clutch for connecting the carrier to the output member, and a second clutch for connecting the ring gear to the output member.
The object indicated above may also be achieved according to a second aspect of this invention, which provides a vehicle rear driving apparatus for driving a hybrid automotive vehicle in a reverse direction, the automotive vehicle having a drive power source consisting of an engine operable by combustion of a fuel and an electric motor, an output member operatively connected to a drive wheel of the vehicle for driving the vehicle, a synthesizing/distributing device of gear type including a housing, a first rotary element, a second rotary element and a third rotary element, the first and second rotary elements being rotated in respective opposite directions when the third rotary element is stationary, and connected to the engine and the electric motor, respectively, and a brake for fixing the third rotary element to the housing, the vehicle rear driving apparatus being characterized in that the brake is a frictional coupling device capable of effecting a slipping engagement, that reverse-motor-drive control means is provided for operating the electric motor in a reverse direction to rotate the second rotary element while the brake is held in its released state, for thereby driving the vehicle in the reverse direction, and that engine-assisted reverse-motor-drive control means is provided, the engine-assisted reverse-motor-drive control means being operable when the vehicle is driven in the reverse direction under the control of the reverse-motor-drive control means, the engine-assisted reverse-motor-drive control means operating the engine to rotate the first rotary element in a forward direction, and controlling the brake to effect the slipping engagement while the third rotary element is rotated in a forward direction with a rotary motion of the first rotary element in the forward direction, so that a rotary motion of the third rotary element is restricted to cause a drive torque to act on the second rotary element in a reverse direction to drive the vehicle in the reverse direction.
In the vehicle rear driving apparatus constructed according to the second aspect of the present invention, the vehicle can be driven in the reverse direction with the electric motor operated in the reverse direction with the brake under the control the reverse-motor-drive control means reverse-motor-drive control means. During this reverse running of the vehicle with the electric motor, an assisting rear drive torque can be produced by the engine under the control of the engine-assisted reverse-motor-drive control means, such that the engine is operated to rotate the first rotary element in the forward direction, while at the same time the brake is controlled to effect a slipping engagement with the third rotary element being rotated in the forward direction with a rotary motion of the first rotary element in the forward direction, so that a rotary motion of the third rotary element is restricted to cause the assisting rear drive torque to act on the second rotary element in the reverse direction to drive the vehicle in the reverse direction. Thus, the vehicle can be driven in the reverse direction by the engine as well as the electric motor, with a sufficiently large total drive force. Further, the slipping engagement of the brake makes it possible to reduce a variation in the vehicle drive force upon switching of the drive mode from the reverse motor drive mode to the engine-assisted reverse motor drive mode, and permits the vehicle to be driven in the reverse direction even at a relatively low speed while maintaining a relatively high operating speed of the drive power source such as an engine. The vehicle rear driving apparatus according to the second aspect of the invention is considered to be one form of the apparatus according to the first aspect of the invention, and the engine-assisted reverse-motor-drive control means of the apparatus according to the second aspect is considered to be one form of the reverse friction-drive control means of the apparatus according to the first aspect.
The electric motor used in the apparatus according to the second aspect of the invention is preferably a motor/generator which functions not only as an electric motor but also as an electric generator.
As described above, the engine-assisted reverse-motor-drive control means is adapted to assist the electric motor during running of the vehicle in the reverse direction under the control of the reverse-motor-drive control means. During this engine-assisted reverse motor drive mode, the electric motor need not be operated to provide its maximum torque, provided the total vehicle drive torque desired by the vehicle operator is by the electric motor and the engine. The proportion of the drive torques produced by the electric motor and the engine may be suitably determined as needed, on the basis of selected vehicle operating parameters such as the amount of electric energy stored in a battery or other electric energy storing device used for the electric motor.
The engine-assisted reverse-motor-drive control means is arranged to control the slip speed or engaging torque of the brake to permit the engine to provide an assisting rear drive force during reverse running of the vehicle by the electric motor. After the vehicle running speed has been raised to a level at which the engine will not stall even with the brake being fully engaged, however, the brake may be brought to its fully engaged state. It is also possible to drive the vehicle in the reverse direction with only the drive torque produced by the engine, by placing the electric motor in a free or non-load state without an output drive force and holding the brake in a slipping state or the fully engaged state. The reverse friction-drive control means provided according to the first aspect of the invention may also be arranged to fully engage the brake when a predetermined condition is satisfied.
The vehicle rear driving apparatus according to the second aspect of the invention described above is advantageously applicable to a hybrid vehicle wherein the synthesizing/distributing device comprises a planetary gear device including a sun gear as the first rotary element, a carrier as the second rotary element, and a ring gear as the third rotary element, the engine being connected to the sun gear while the electric motor being connected to the carrier. In this form of the invention, the brake is operable to fix the ring gear to the housing, and the apparatus includes a first clutch for connecting the carrier to the output member, and a second clutch for connecting the ring gear to the output member.
According to a first preferred form of the second aspect of the invention described above, the vehicle rear driving apparatus further comprises mode determining means for selecting an operating mode of the vehicle such that an engine-assisted reverse motor drive mode in which the vehicle is driven in the reverse direction by the electric motor and the engine under the control of the engine-assisted reverse-motor-drive control means is selected more frequently when an amount of electric energy stored in an electric energy storing device to operate the electric motor is relatively small than when the amount of electric energy stored in the electric energy storing device is relatively large.
In the first preferred form of the apparatus according to the second aspect of this invention wherein the engine-assisted reverse motor drive mode is established more frequently when the electric energy amount stored in the electric energy storing device is relatively small than when the stored electric energy amount is relatively large. This arrangement results in a reduced frequency of operation of the electric motor and a reduced amount of electric energy consumption by the electric motor, since the engine is more frequently operated to assist the electric motor when the stored electric energy amount is relatively small. Thus, the electric energy can be more effectively utilized. In other words, the mode determining means relatively frequently selects the engine-assisted reverse motor drive mode (relatively frequently activates the engine-assisted reverse-motor-drive control means) when the stored electric energy amount is relatively large, and relatively infrequently selects a reverse-motor-drive mode in which the vehicle is driven in the reverse direction by only the electric motor (relatively infrequently activates the reverse-motor-drive control means) when the stored electric energy amount is relatively large. Accordingly, a relatively large amount of electric energy is consumed by the electric motor under the control of the reverse-motor-drive control means when the stored electric energy amount is relatively large. When the stored electric energy amount is relatively large, on the other hand, the engine is relatively frequently operated to assist the electric motor, and the operation of the electric motor is restricted, so that a relatively small amount of electric energy is consumed by the electric motor under the control of the engine-assisted reverse-motor-drive control means when the stored electric energy amount is relatively small. Thus, the required vehicle drive torque can be obtained while minimizing the amount of consumption of the electric energy, so that the electric energy amount stored in the electric energy storing device can be maintained in a range in which the energy consumption efficiency is relatively high.
For instance, the mode determining means is adapted to select the engine-assisted reverse motor drive mode or activates the engine-assisted reverse-motor-drive control means, when the operator""s desired vehicle drive torque is larger than the permissible maximum output torque of the electric motor. In this instance, the permissible maximum output torque may be determined such that the permissible maximum output torque decreases with a decrease in the amount of electric energy stored in the electric energy storing device. This arrangement results in a relatively high frequency of operation of the engine as well as the electric motor when the stored electric energy amount is relatively small.
According to a second preferred form of the apparatus according to the above second aspect of the present invention, the engine-assisted reverse-motor-drive control means includes means for controlling an engaging torque of the brake according to a difference between a desired vehicle drive torque desired by an operator of the vehicle and an output torque of the electric motor, and controls an output torque of the engine in a feedback fashion such that an operating speed of the engine coincides with a desired value.
In the second preferred form of the apparatus according to the second aspect of the invention, the engaging torque of the brake is controlled depending upon an amount of shortage of the actual total vehicle drive force, which is a difference of the operator""s desired drive force from the output torque of the electric motor. Further, the output torque of the engine is feedback-controlled such that the speed of the engine coincides with the desired value. Thus, the operator""s desired drive torque can be produced by both of the engine and the motor/generator, with the engine speed being controlled to the desired value with high accuracy.
The engine-assisted reverse-motor-drive control means provided according to the above-described second preferred form of the apparatus may be adapted to control the engaging torque of the brake while taking into account the gear ratio of the synthesizing/distributing device such that the drive torque corresponding to the difference between the operator""s desired vehicle drive torque and the output torque of the electric motor acts on the second rotary element through the slipping engagement of the brake. At the same time, the engine-assisted reverse-motor-drive control means controls the output torque of the engine so that the engine speed is made equal to the desired value. For the feedback control of the engine torque, it is desirable to control as a feed-forward item a portion of the engaging torque of the brake which corresponds to the engine, namely, the drive torque corresponding to the gear ratio of the synthesizing/distributing device.
Although the output torque of the engine is feedback-controlled according to the second preferred form, the engine-assisted reverse-motor-drive control means according to the second aspect of this invention may be adapted to control the engine torque depending upon the difference between the operator""s desired drive torque and the output torque of the electric motor, and at the same time feedback-control the engaging torque of the brake such that the engine speed coincides with the desired value. Namely, the engine torque is controlled while taking into account the gear ratio of the synthesizing/distributing device such that the drive torque equal to the above-indicated difference acts on the second rotary element, and at the same time the engaging torque for the brake is controlled in a feedback fashion such that the engine speed coincides with the desired value. For the feedback control of the engaging torque of the brake, it is desirable to control as a feed-forward item a portion of the engine torque which corresponds to the brake, namely, the drive torque corresponding to the gear ratio of the synthesizing/distributing device.